Chemistry and oxidation capacity of the nitrate radical in the continental boundary layer near Berlin

The nitrate radical is in many situations the most important nighttime oxidizing species, removing, for example, hydrocarbons, which would otherwise be available to daytime ozone formation. In spite of its importance in the night and probably also under certain conditions during the day, our understanding of the NO3 chemistry and its impact on the oxidation capacity of the atmosphere is still incomplete. Here we present measurements of NO3 by differential optical absorption spectroscopy (DOAS) and a number of other atmospheric trace gases performed during the Berliner Ozonexperiment (BERLIOZ) campaign at Pabstthum near Berlin, Germany, to quantify the contribution of NO3 to the atmospheric oxidation rate of volatile organic compounds (VOCs) and NO, removal. The measurements show that only two NO3 sinks were of importance: (1) About 50 30% (depending on the distance (0.1 - 3 km) to a near forest) of the NO3 was lost due to reaction with biogenic hydrocarbons. (2) The major part of the remaining loss probably can be attributed to the indirect loss via the reaction of N2O5 on aerosol surfaces. Assuming that heterogeneous hydrolysis of N2O5 is occurring, the nonphotolytical conversion of NO, to HNO3 via N2O5 was found to be comparable with daytime conversion by the reaction of OH with NO2. In combination with measurements of the OH concentration, it was possible for the first time to derive a relative contribution of 28% (24-hour average) for the NO3-initiated oxidation to the total VOC degradation.